Tubby Vs New Audi RS4 (420bhp)

[Andy Champ]

You need to read this whole thread mate. Its about power to weight but not BHP to weight. Acceleration will be mostly down to Torque@Revs to weight.

Thats what the whole discussion has been about.

Scott =op

Scott,

You are mixing power (which is measured in watts, or kilowatts, or BHP) with force (which is measured in newtons, or pounds-force).

Refer to:
http://en.wikipedia.org/wiki/Power_%28physics%29
http://en.wikipedia.org/wiki/Motive_power
http://en.wikipedia.org/wiki/Force

You might also like to do a dimensional analysis on the classic equation
E=1/2*MV^2
(Energy = half Mass times velocity squared)

Andy

[Scotster]

Power isn't measured in BHP though

Thats where what u are saying confuses me.

Scott =op

[Bibbs]

You need to read this whole thread mate. Its about power to weight but not BHP to weight. Acceleration will be mostly down to Torque@Revs to weight.

Thats what the whole discussion has been about.

Scott =op

The area under a Torque/Speed( or revs) curve .. then factor in the weight.

F = MA
Area of the curve = Mass x Acceleration.

No mention of BHP in Newton's Second law.

[Andy Champ]

Power isn't measured in BHP though

Thats where what u are saying confuses me.

Scott =op

Eh?

BHP is short for Brake Horse Power.

The area under a Torque/Speed( or revs) curve .. then factor in the weight.

F = MA
Area of the curve = Mass x Acceleration.

No mention of BHP in Newton's Second law.

In order to develop the same F at a higher speed you need to input more power.

That's why, although momentum is MV, kinetic energy is ½MV².

So... area under a power/speed curve.

Andy

[Scotster]

Power isn't measured in BHP though

Thats where what u are saying confuses me.

Scott =op

Eh?

BHP is short for Brake Horse Power.

The area under a Torque/Speed( or revs) curve .. then factor in the weight.

F = MA
Area of the curve = Mass x Acceleration.

No mention of BHP in Newton's Second law.

In order to develop the same F at a higher speed you need to input more power.

That's why, although momentum is MV, kinetic energy is ½MV².

So... area under a power/speed curve.

Andy

One minute you seem to be very intelligent when it comes to the power transfer in a car and the next it seems like you don't have a clue. BHP is not power regardless of the use of the word "Power". You have already stated the units of power and i can assure you BHP isn't one of them. BHP is a calculation not an actual figure. It is guessed by a calculation of an actual figure. I'll tell you what, what is the unit of measurement of BHP? I'll save u the time, the unit of measure for BHP is BHP. Its a calculation not a measurement regardless of where it is taken.

In fact, someone can correct me if i'm wrong, i'm pretty sure BHP is a measurement of how many "horses" it would take to stop the flywheel from spinning. The peak is when it takes the most "horses" to stop the flywheel. A "horse" is an predetermined constant. Of course this may be total bull$h1t but its what i have read elsewhere.

I think this discussion has went on long enough. People who know far more about it that you and i have already posted and i am very satisfied with their answer. If you want to say that BHP is in fact a measure of Power then thats up to you. IMO thats a lot of crap and makes everything intelligent that you have said irrelevant but we are getting nowhere arguing about it.

Scott =op

[Andy Champ]

One minute you seem to be very intelligent when it comes to the power transfer in a car and the next it seems like you don't have a clue. BHP is not power regardless of the use of the word "Power". You have already stated the units of power and i can assure you BHP isn't one of them. BHP is a calculation not an actual figure. It is guessed by a calculation of an actual figure. I'll tell you what, what is the unit of measurement of BHP? I'll save u the time, the unit of measure for BHP is BHP. Its a calculation not a measurement regardless of where it is taken.

In fact, someone can correct me if i'm wrong, i'm pretty sure BHP is a measurement of how many "horses" it would take to stop the flywheel from spinning. The peak is when it takes the most "horses" to stop the flywheel. A "horse" is an predetermined constant. Of course this may be total bull$h1t but its what i have read elsewhere.

I think this discussion has went on long enough. People who know far more about it that you and i have already posted and i am very satisfied with their answer. If you want to say that BHP is in fact a measure of Power then thats up to you. IMO thats a lot of crap and makes everything intelligent that you have said irrelevant but we are getting nowhere arguing about it.

Scott =op

I'm beginning to suspect you're trolling.

Read this: http://www.howstuffworks.com/fpte.htm

Andy

[Scotster]

Read the information on "Power" Andy.

As i have said too many times. Its a calculation. The power is calculated not measured. What part of this do you not get?? The torque is measured, and the BHP is calculated from that taking into the equation time and revs (on a car).

Its STILL a calculation and NOT a measurement. When your car goes onto a dyno the Torque is measured @ Revs and the BHP is calculated from that along with some other info. Try reverse engineering the whole calculation of BHP with regards to a car and you will end up with what is measured. I can assure you BHP isn't measured, its calculated.

Good grief!

Scott =op

[Andy Champ]

Read the information on "Power" Andy.

Is that "I have read" or "You should read"?

As i have said too many times. Its a calculation. The power is calculated not measured. What part of this do you not get?? The torque is measured, and the BHP is calculated from that taking into the equation time and revs (on a car).

Its STILL a calculation and NOT a measurement. When your car goes onto a dyno the Torque is measured @ Revs and the BHP is calculated from that along with some other info. Try reverse engineering the whole calculation of BHP with regards to a car and you will end up with what is measured. I can assure you BHP isn't measured, its calculated.

Good grief!

Scott =op

OK, I'm not familiar with the way all dynamometers work. My understanding is that most of the ones used on cars are flywheel units. In that case, both torque and power are deduced from the change in speed over time. But I could easily be wrong on that. The ones used on railways, for example, measure tractive effort and speed, and I'm sure that isn't what goes on when you are on the rollers. Feel free to give me an engineering lecture on how dynos work.

However, that's beside the point...

What units do you think power is measured in? Please give both SI (metres/kilograms/seconds etc) and Imperial (foot/pounds/seconds etc)

Andy

[skinthespin]

Id have thought the only thing you could actually measure at the wheels is the tractive effort, the torque, and hence the power is converted back from that.........

[Andy Champ]

Id have thought the only thing you could actually measure at the wheels is the tractive effort, the torque, and hence the power is converted back from that.........

Dunno about you, but the only device in my car measures speed. It's kind of important too.

Andy

[skinthespin]

On a rolling road I meant, it measures speed and the tractive effort, from that calculates back to give you torque and power.

[Bibbs]

http://www.ktrperformance.com/dyno_tuning/dyno_faq.htm

How is engine power measured?
The fact is there is no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the foundation of all engine design, development, and tuning. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. Read on to find out more about these two methods.

[Scotster]

Thankyou Bibbs.

I was too lazy to search. Thats exactly what i was getting at.

On a rolling road, as skin said, torque isn't exactly a direct measurement. That is why hub measurement is usually taken as being more accurate. The torque is almost a direct measurement on a rolling road as the only thing that will be taken into the equation is the rad of the wheel/tyre combo giving the force times the distance as it takes more torque to turn a larger wheel (obviously).

In 5th gear my 130bhp TDI would absolutely HORSE my friends 150bhp petrol from 60 to around 110. Then his car would catch and pass as his BHP was a good bit higher. Both exactly the same cars, Vectra SRI's. Same age too with same size wheels. Of course this is because the torque of my car was much higher at lower revs. Even with the disadvantage of a diesel engine reving slower, it still beat him because of the difference in torque.

Scott =op

[Andy Champ]

http://www.ktrperformance.com/dyno_tuning/dyno_faq.htm

How is engine power measured?
The fact is there is no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the foundation of all engine design, development, and tuning. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. Read on to find out more about these two methods.

Another quote from the same page (Great link BTW, thanks)

"The majority of rolling roads used to determine power figures in the US are inertia dynamometers. Inertia dynos do not directly measure the force on the dyno rollers to determine power figures. Instead, these systems calculate the force on the rollers using the formula:
F = ma
Where F is force; m is mass; a is acceleration"

They don't measure the torque, or the tractive effort, or the power. They measure the acceleration. Given the (known) moment of inertia of the roller system, they can all be calculated.

In 5th gear my 130bhp TDI would absolutely HORSE my friends 150bhp petrol from 60 to around 110. Then his car would catch and pass as his BHP was a good bit higher. Both exactly the same cars, Vectra SRI's. Same age too with same size wheels. Of course this is because the torque of my car was much higher at lower revs. Even with the disadvantage of a diesel engine reving slower, it still beat him because of the difference in torque.Scott =op

Ah, now we have a good comparison.

Firstly, you've said that his higher BHP helps.

However, at the time you are talking about, where your car is faster than his, what gear were you each in? What engine revs were you running at?

Andy

[Scotster]

http://www.ktrperformance.com/dyno_tuning/dyno_faq.htm

How is engine power measured?
The fact is there is no way of directly measuring power - all types of dynamometer measure torque and then power is calculated from the formula BHP = Torque (ft/lbs) x rpm/5252. This basic equation is the foundation of all engine design, development, and tuning. Two main methods of measuring power are used in the automotive industry - (1) measurement at the crankshaft of the engine or (2) measurement at the driving wheels. Read on to find out more about these two methods.

Another quote from the same page (Great link BTW, thanks)

"The majority of rolling roads used to determine power figures in the US are inertia dynamometers. Inertia dynos do not directly measure the force on the dyno rollers to determine power figures. Instead, these systems calculate the force on the rollers using the formula:
F = ma
Where F is force; m is mass; a is acceleration"

They don't measure the torque, or the tractive effort, or the power. They measure the acceleration. Given the (known) moment of inertia of the roller system, they can all be calculated.

In 5th gear my 130bhp TDI would absolutely HORSE my friends 150bhp petrol from 60 to around 110. Then his car would catch and pass as his BHP was a good bit higher. Both exactly the same cars, Vectra SRI's. Same age too with same size wheels. Of course this is because the torque of my car was much higher at lower revs. Even with the disadvantage of a diesel engine reving slower, it still beat him because of the difference in torque.Scott =op

Ah, now we have a good comparison.

Firstly, you've said that his higher BHP helps.

However, at the time you are talking about, where your car is faster than his, what gear were you each in? What engine revs were you running at?

Andy

Can't say what gear/revs any of us were at but i would say between 4000 and 5000 for me and between 5500 and 6500 for him. We were both absolutely gunning it.

I have always said that BHP would make a difference at the top end. I mearly said when comparing 2 cars off the line torque has more of a hand in it than BHP. I also said that BHP was not measured, it was mearly calculated so a change in torque would directly relate to a change in BHP as its calculated from it.

Also, there seems to be an awful lot of rolling road talk here. I only mentioned the rolling road to prove that BHP was a calculation and not a direct measurement. On a rolling road it is still torque that is measured, from the rollers instead of the car though. Then the torque of the car is calculated from the information.

Your never going to give in and i'm getting fed up saying the same things over and over. This thread has went from off topic to way off topic to off the scale off topic. It was very interesting for a while reading all the different views and digesting all the relevant information but even the information you provided backed up my views.

Scott =op

[Scotster]

Id have thought the only thing you could actually measure at the wheels is the tractive effort, the torque, and hence the power is converted back from that.........

Dunno about you, but the only device in my car measures speed. It's kind of important too.

Andy

Does the device that measures the speed not come off of the gearbox?

Didn't think it came off the wheels :p

Scott =op

[skinthespin]

lol@this thread.

surely the only thing a rolling road can measure is the force from the contact patch spinning the rollers, that force is the tractive effort from the car. The computer in the dyno knows how big the rollers are, so converts this force into a torque, which it can then convert into power, no?

[Scotster]

Well yeah that sounds exactly what it does. But, the tractive effort will be a calculation too. The torque and acceleration of the rollers will be a direct measurement. From that it should take into account the size and weight of the rollers. I don't know what calculation will be used to measure the torque at the flywheel though. THis would only give you the torque at the rear wheels as it would be equal to that of the rollers when the size and weight of them is taken into account.

To summarise:

The tractive effort on the rollers will be calculated by measuring the torque applied to the rollers, along with the acceleration, and then taking into account the size and weight of them to work out how much force is being applied to make them turn @ x revs. This would give the torque graph. The power one is then calculated from this.

Scott =op

[skinthespin]

Im bored now, RS4's are fast.

[EarL]

Im bored now, RS4's are fast.

...and torquey.